Only generate additional properties at base class level

[sdecoodt]

Fixes #1488

The JsonExtensionData attribute is only allowed once on a class.
A previous fix to make sure it's only generated once (#1447) did not cover deep hiarachies of inheritance.

• Included a unit test to reproduce the multilevel inheritance issue
• Extended the model so that there is a sepperation of logic
  • Indication if the class has addtional properties
  • Indication if the additional properties actually need to be generated

Related Issues:
#2818 on the NSwag repo

[RicoSuter]

Thanks for the PR.

[sdecoodt]

Thanks for the PR.

You're welcome. Thanks for the merge.
I realize it was a bit of a selfish contribution :)
I'm willing to help tackle something else if you have suggestions.